1. Field of the Invention
The present invention relates generally to devices for distributing offset preventing material to the peripheral surface of a fixing roller, and more particularly to apparatus for applying such material without direct contact to the roller by intermediate feed devices.
2. Description of the Prior Art
In the process of xerography, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided developing material or electrostatic marking particles (toner) which is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed toner image is usually transferred to the surface of a support material such as paper, to which it may be fused by heat or vapor fixing techniques. During such fusing, the individual toner particles soften and become tacky so that they readily adhere to the surface of the support material.
It is common practice in the art to apply heat for fusing the toner image to paper by bringing the toner image into direct contact with a hot surface, such as the peripheral surface of a heated roller such as for example the roller disclosed in U.S. Pat. No. 3,437,032, which issued Apr. 8, 1969 to A. T. Manghirmalani et al. However, since the toner image is tackified by heat, part of the image carried by the support material will stick to the surface of the heated roller so that, as a second sheet is brought into contact with that surface, the tackified image partially removed from the first sheet will partly transfer to the second sheet and, at the same time, part of the tackified image from the second sheet will itself adhere to the heated roller. Also, during revolution of rollers 12 and 14 (FIG. 1) without paper therebetween, toner on the fusing roller may transfer to the pressure roller. These conditions are commonly referred to in the printing art as "set off" or "offset," the latter term being preferred and used throughout the remainder of the present specification.
The offset of toner onto the heated roller has led to the use of other types of fixing devices, such as coiled radiant element heaters with reflectors. However, such devices have the disadvantages of dissipating a large quantity of heat into the machine enclosure, insufficiently heating the toner image and presenting a safety hazard because of the exposed radiant element. Therefore, improved methods for direct contact fusing devices which will rapidly fuse toner images without causing the toner particles to adhere to the fusing roller while in a tackified state have been developed.
One such method involves direct fusing devices in which the fusing roller is coated with an abhesive material such as tetrafluoroethylene resin sold under the trademark Teflon by the DuPont Corporation or silicone rubber or elastomer. Such rollers may be provided with an offset preventing film such as a silicone oil film to prevent toner offset. Teflon, silicone rubber and silicone oil have physical characteristics such that they form a "release" surface which is highly repellent to tacky substances.
The silicone oil film may be applied to the fusing roller by various techniques. For instance, U.S. Pat. No. 3,132,047, which issued May 5, 1964 to W. G. Van Dorn, discloses the use of an applicator roller to transfer oil from a pan directly to the surface of a fusing roller. In U.S. Pat. No. 3,256,002, which issued June 14, 1966 to F. W. Hudson, a wick is provided between the applicator roller and the fusing roller surface, and in British Pat. No. 1,399,740 published July 2, 1975, an oil pump was substituted for the applicator roller of the Hudson patent. In these and every other prior art method known to us for applying offset preventing material to fusing rollers, wicks and/or applicator rollers contact the fusing roller. Such contact tends to abrade the adhesive surface of the fusing roller resulting in premature failure of the roller.
Further, prior art wicking apparatus require low viscosity offset preventing liquids. For instance, Dow-Corning's FS 1265 fluorosilicone oil, having a viscosity of 1,000 centistokes, has been wicked onto a thermally conductive silicone elastomer. Although such an oil functions well for prevention of offset, it is expensive, it adversely affects the physical properties of silicone elastomers, it deteriorates the bond between the silicone elastomers and aluminum roller cores and its degradation products are toxic.
Dimethyl silicone oil such as Dow-Corning's DC 200 is a logical replacement for fluorosilicone oil, but at viscosities similar to FS 1265, it excessively swells silicone rubbers. We have found that higher viscosity silicone oils have considerably less effect on silicone elastomers, but wicking such high viscosity oils (i.e., oils having viscosities of at least 10,000 centistokes) presents additional problems. That is, wicks may occasionally be dismembered upon start-up or may clog after long use of high viscosity oils. Besides having less effect on the physical properties of silicone elastomers. silicone oils with viscosities up to about 100,000 centistokes provide better release characteristics for a given quantity of oil and greater persistence on the fusing and pressure rollers.